Air-in-line and pressure detection

ABSTRACT

Methods and apparatus for air content and pressure measurement of sample fluid, especially sample fluid in association with an infusion pump. Volume change in a chamber as the chamber transitions between negative and positive pressure relates to the air content in the chamber. In particular, in an infusion pump, the volume change of infusion fluid as it transitions between being under negative pressure and positive pressure within a cassette central chamber, e.g., pumping chamber, relates to the air content in the infusion fluid. The outlet pressure of the cassette central chamber, e.g., blood pressure, can be monitored based on the cassette central chamber pressure.

This application is a divisional of U.S. patent application Ser. No. 09/899,300 filed on Jul. 6, 2001, entitled “Air-In-Line and Pressure Detection,” now issued as U.S. Pat. No. 7,232,430, which is based on and claims priority from U.S. Provisional Patent Application No. 60/216,772, filed on Jul. 7, 2000, the entire subject matter of these applications being incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system and methods of determining air content and pressure in fluid, especially in association with an infusion pump.

BACKGROUND OF THE INVENTION

There is a need in the field for methods of measuring air content and pressure of a sample fluid. Particularly in an infusion pump, it is critical to determine the air content of the infusion fluid and the fluid pressure down stream of the outlet valve of the infusion pump.

SUMMARY OF THE INVENTION

The present invention is directed in part to unique methods of content measurement of a sample fluid. The present invention is based on the discovery that volume change in a chamber, as the chamber transitions between negative and positive pressure relates to the air content in the chamber. In particular, in an infusion pump, the volume change of infusion fluid as it transitions between being under negative pressure and positive pressure within a cassette central chamber, e.g., pumping chamber, relates to the air content in the infusion fluid. In addition, the present invention provides methods for determining pressure of a sample fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of an exemplary embodiment of the invention taken in combination with the accompanying drawings, of which:

FIG. 1 is a block diagram illustrating the fluid delivery system topology of an infusion pump.

FIG. 2 is a block diagram illustrating a cross-sectional view of the cassette assembly of an infusion pump.

FIG. 3 illustrates changes in cassette central chamber volume as a function of pressure.

FIG. 4 illustrates changes in piston position as a function of time.

DETAILED DESCRIPTION

Infusion pumps are widely used for administering medications to patients over an extended time period. During an infusion of medication, it is critical to monitor the air content of the fluid medication administered to a patient. In addition, it is often convenient/helpful to measure the pressure on the patient side of the pump, e. g. measure the blood pressure of the patient. One of the applications of the methods for air content measurement of a sample fluid is to measure the air content in a cassette central chamber in an infusion pump. In addition, the mechanism for air content measurement also provides means to monitor the blood pressure of a patient connected to an infusion pump.

FIG. 1 is a block diagram illustrating one embodiment of the present invention. The fluid delivery system 100 includes a cassette assembly 20 and a shuttle mechanism 40. A suitable cassette assembly is described in patent application No. 60/216,658, filed Jul. 7, 2000, entitled “Cassette”, to Carlisle, Costa, Holmes, Kirkman, Thompson and Semler, the entire contents of which are incorporated herein by reference. Within the cassette assembly 20 is a cassette piston 60 and a cassette central chamber 80. A spring 120 biases shuttle mechanism 40 which is connected to the cassette piston 60. Piston 60 slides freely in the cassette central chamber 80 to draw fluid into central chamber 80 and pump fluid out of central chamber 80. A motor 140 is activated in one direction to draw the cassette piston 60 out of cassette central chamber 80 via cam 160 and shuttle 40. When the cassette piston 60 is fully withdrawn, shuttle 40 disengages from cam 160 and motor 140, so that spring 120 pushes the cassette piston 60 into the cassette central chamber 80 via shuttle 40 to apply positive pressure to the fluid in the cassette central chamber 80. The shuttle mechanism 40 is also operably linked to an optical position sensor 180. A suitable position sensor is described in patent application No. 60/217,885, filed Jul. 7, 2000, entitled “Optical Position Sensor and Position Determination Method”, to Carlisle, Kaplan and Kirkman, the entire contents of which are incorporated herein by reference. A processor 220 is connected to motor 140 and the position sensor 180.

FIG. 2 is a diagram illustrating a cross-sectional view of a cassette assembly 20. The cassette assembly 20 contains an inlet valve 200, an outlet valve 210, a cassette central chamber 80, and a cassette piston 60. Cassette piston 60 is connected to shuttle 40.

In operation, the motor 140 is activated in one direction to withdraw the cassette piston 60 against the force of spring 120 via cam 160, creating a relative vacuum in the cassette central chamber 80 and pulling fluid through a one-way passive inlet valve 200 into the cassette central chamber 80. During this fill stroke, the pressure in the cassette central chamber 80 is negative, e.g., between 0 and −10 psi. The amount of negative pressure depends on the withdrawal speed of the piston, fluid resistance, fluid viscosity, etc. Once the cassette piston 60 has been withdrawn, cam 160 disengages from the shuttle 40, enabling the spring mechanism 120 to urge shuttle 40 to drive piston 60 into the cassette central chamber 80. The pressure in the chamber then moves from a negative value through zero to a positive value. The one-way passive inlet valve 200 is now fully closed. The positive pressure in the cassette central chamber 80 is typically between +2 and +7 psi depending on the spring force applied to the cassette piston 60 through the shuttle 40 which is directly related to the length of the withdrawal stroke, e.g., the further the withdrawal stroke the stronger the spring force.

In a closed cassette central chamber, the volume changes as a function of cassette central chamber pressure. In theory, when the cassette central chamber 80 is closed and contains only liquid, i.e., air free fluid, the cassette central chamber is not compressible, thus no volume change occurs. Nevertheless in practice, a “base volume change” exists when the chamber contains just air free fluid (as shown in FIG. 3). Such “base volume change” is irrelevant to the air content in the fluid and is mostly due to system designs such as the shape of a sealing member of the cassette piston 60 or the flexing of elastomeric inlet and outlet valve elements connected to the cassette central chamber 80.

For example, the cassette piston 60 in the cassette central chamber 80 acts as a nearly ideal piston when under positive pressure from the spring mechanism 120; thus a change in the axial position of the piston represents a fluid volume change in the cassette central chamber. Nevertheless, when cassette central chamber pressure changes from negative to positive, the shape of a sealing member of the cassette piston changes and results in piston travel without any change in central chamber fluid volume. This amount of travel contributes to the “base volume change”; it is significant, however, that this travel is a relative constant of the system design and does not change over time. In addition, the elastomeric valve elements connected to the cassette central chamber have some inherent displacement determined by their geometry and material properties. When under pressure, these elements move and to an insignificant degree, continue to move (creep) over time. The movement of these elements also contributes to the “base volume change”.

The “base volume change” of the cassette central chamber 80 can be determined by detecting the volume change of a control fluid under the pressure change of the cassette central chamber. The volume change can be measured by determining the change of shuttle position, i.e., the piston travel position when the cassette central chamber pressure changes from negative to positive. The change of shuttle position is determined by the precision position sensor 180 linked to the shuttle mechanism 60. For example, one can compare the shuttle position in two states: the peak position during piston withdrawal and the shuttle position after the spring pressure is applied (as shown in FIG. 4). This net displacement change of the shuttle position as a result of the pressure change in the cassette central chamber 80 filled with control fluid is a measure of the “base volume change” of the system 100, e.g., the volume change that is inherent in the system 100.

In one embodiment, the “base volume change” of a control fluid is determined more than once and statistically conservative limits of “base volume change”, e.g., lower than average, is selected as the “base volume change” for calculating the fluid air content. In another embodiment, the base volume change of a control fluid for a sample infusion fluid is determined by measuring the median volume change of more than one sample of the same infusion fluid, e.g., over more than one fill stroke, resulting from the pressure change of the cassette central chamber. In yet another embodiment, the base volume change is the median volume change of an infusion fluid over eleven (11) contiguous fill strokes, and is updated or modified periodically throughout an infusion therapy; and such base volume change is used to measure the sample fluid air content of the same infusion fluid.

In a closed cassette central chamber, e.g., both inlet and outlet valves are closed, any cassette central chamber volume-versus-pressure changes above the “base volume change” are interpreted as volume changes in the cassette central chamber due to the presence of air (as shown in FIG. 3). The air content of fluid contributes to the total volume change of the cassette central chamber and is proportional to the total volume change, e.g., sample volume change minus the base volume change.

For example, one can fill the cassette central chamber 80 with a sample fluid until the volume (gVolMax) in the chamber is greater than the desired volume of fluid to be delivered in a single pump stroke (gVdue) plus the “base volume change” (Vbase). During the fill cycle, the fill volume can be monitored through the piston position, i.e., the shuttle position which is determined by the optical position sensor 180. As a result of the geometry and design of the cassette assembly 20, there is a linear relationship between the shuttle position and the fill volume. Once the fill volume (gVolMax) is achieved, the motor direction is reversed so that the shuttle 40 falls off the cam 160 and rides freely on the spring mechanism 120. Similarly the end-diastolic volume (gVolEnd) can be determined from the stabilized shuttle position after the cam 160 releases shuttle 40. The sample volume change is the difference between the gVolMax and gVolEnd. The air content of the sample fluid is calculated in processor 220 as follows: Air content (Volair)˜Sample volume change−Base volume change ˜(gVolmax−gVolEnd)−Base volume change

The air content measured according to the present invention is independent of the size and shape of the air bubble contained in a sample fluid, e.g., the air content includes the content of big bubbles, small bubbles, integrated bubbles, and unintegrated bubbles.

In one embodiment, the effective amount of fluid that is pumped out of cassette central chamber 80 is calculated based on the air content of a sample fluid. For example, for a given stroke, the effective amount of fluid that is infused is calculated in processor 220 as follows: effective amount of fluid infused˜gVolmax−Volair

In another embodiment, the proportion of air content and volume content in a given stroke is calculated directly on the change of position of the piston. Specifically the proportion is calculated in processor 220 as follows: proportion of air/fluid content=(position change due to pressure change)/(max. position under neg. pressure)

The processor 220 adjusts subsequent fluid flow rate based on the air/fluid content proportion in a given stroke to compensation for the air content detected in a sample fluid.

In yet another embodiment, processor 220 compares the air content of a sample fluid to a predetermined value stored in the processor 220; the processor 220 activates an alarming device if the air content of the sample fluid is close to or beyond the predetermined value. Alternatively, the processor 220 activates an alarming device as well as shuts down the out flow of sample fluid from the cassette central chamber 80, e.g., closes the outlet valve 210 of the cassette central chamber 80 and shuts down infusion process by the fluid delivery system 100.

According to another feature of the invention, sample fluid continuously passes through cassette central chamber 80 and the air content of the sample fluid is determined at different time points and stored in processor 220. The processor 220 calculates accumulated air content of the sample fluid by adding the air content values collected at different time points. Such accumulated air content over a period of time is compared to a threshold air content value stored in the processor 220; the processor 220 triggers a notifying device, e.g., an alarm, if the accumulated air content is close or beyond a predetermined limitation. Alternatively, the processor 220 activates a notifying device as well as shuts down the out flow of sample fluid from the cassette central chamber 80, e.g., closes the outlet valve 210 of the cassette central chamber 80 and shuts down infusion process by the fluid delivery system 100.

In one embodiment, the outlet pressure of the cassette central chamber, e.g., the blood pressure of a mammal such as a human connected to the infusion pump is monitored. For example, during the fluid displacement, the outlet valve 210 of the cassette central chamber 80 is opened via external actuation. Fluid then flows from the higher pressure in the cassette central chamber 80 to the outlet via the outlet valve 210. If the outlet valve 210 remains open, the cassette piston 60 will stop when the cassette central chamber pressure equals the outlet pressure. The position of the piston on the spring load is associated with a known spring force. Processor 220 then calculates the outlet pressure from the fixed geometry of the cassette central chamber 80. With the outlet valve 210 open, the outlet pressure including even rapid changes in arterial, vein, or capillary pressure of a patient can be measured.

For example, the spring rate, k (in units of force/distance), of the shuttle mechanism 40 and the piston cross-sectional area, A, are known system design constants. Such system design constants, i.e., k/A are pre-calculated and stored in the processor 220 as Design_Constant. During the empty cycle, the outlet valve 210 of the cassette central chamber remains open. Once the spring 120 reaches a stabilized position, the system reaches equilibrium, e.g., the outlet pressure equals the cassette central chamber pressure. Subsequently the shuttle position, i.e., X, is measured by the optical position sensor 180 and processor 220 calculates the outlet pressure as the following: Outlet pressure=Design_Constant*X

In another embodiment, processor 220 monitors the outlet pressure of the cassette central chamber and compares it to a predetermined value over a period of time. An increase of the outlet pressure indicates a partial or complete blockage of the cassette central chamber outlet, i.e., blockage of the outlet pathway or a body fluid pathway receiving fluid displaced from the cassette central chamber 80. Depending on the degree of outlet pressure increase, processor 220 generates a signal to either alert the pressure increase or stop the fluid displacement of the system 100.

Other Embodiments

Although several exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. 

1. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the chamber has an inlet valve, an outlet valve, and an assembly connected to a pressure source, and the assembly comprises a cassette piston.
 2. The method of claim 1, wherein the pressure source comprises a motor connected to the piston and generates negative pressure.
 3. The method of claim 1, wherein the pressure source comprises a spring connected to the piston and generates positive pressure.
 4. The method of claim 1, wherein the sample fluid flows into the chamber the pressure source generates negative pressure.
 5. The method of claim 1, wherein the chamber is part of an infusion pump.
 6. The method of claim 1, wherein the inlet valve and the outlet valve are closed when the pressure source generates positive pressure.
 7. The method of claim 1, wherein the determination of the base volume change and the sample volume change includes determining a position of the cassette piston.
 8. The method of claim 7, wherein the determination of the position of the cassette piston includes using an optical position sensor.
 9. The method of claim 1 further comprising adjusting a fluid flow rate through the chamber based on an air/fluid content proportion in a given stroke of the cassette piston to compensate for the air content detected in the sample fluid.
 10. The method of claim 1 further comprising determining an effective amount of fluid that is pumped out of the chamber based on the determined air content.
 11. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the chamber is closed at the positive pressure.
 12. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the determination of the base volume change is repeated for one or more times and a lower than average value is selected as the base volume change.
 13. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the air content of the sample fluid is related to the sample volume change minus the base volume change.
 14. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and calculating an accumulated air content of more than one sample fluids over a period of time.
 15. The method of claim 14 further comprising comparing the accumulated air content to a predetermined value.
 16. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the determining of the base volume change comprises determining a median base volume change with more than one sample of an infusion fluid.
 17. The method of claim 16, wherein the determining of the median base volume change comprises determining a median base volume change with 11 contiguous samples of an infusion fluid.
 18. The method of claim 16, wherein the sample fluid is a sample of the infusion fluid.
 19. An apparatus for air content measurement of a sample fluid comprising: a central chamber; an assembly moving in and out of the central chamber; a pressure source connected to the assembly, and wherein the pressure source comprises a motor which generates a negative pressure in the central chamber and pumps fluid into the central chamber via the assembly, and a spring which generates a positive pressure in the central chamber and pumps fluid out of the central chamber via the assembly; a position sensor connected to the assembly, wherein the position sensor determines a position change of the assembly; and a processor connected to the position sensor, wherein the processor receives the position change of the assembly, calculates a base volume change between zero or a negative pressure and a positive pressure in the central, and a sample volume change between zero or a negative pressure and the positive pressure in the central chamber with a sample fluid, wherein the air content of the sample fluid is related to the sample volume change minus the base volume change.
 20. The apparatus of claim 19, wherein the assembly is a cassette piston.
 21. The apparatus of claim 19, wherein the central chamber comprises an inlet valve and an outlet valve.
 22. The apparatus of claim 21, wherein the inlet valve and the outlet valve are closed when the pressure source generates positive pressure.
 23. The apparatus of claim 19, wherein the central chamber is part of an infusion pump.
 24. The apparatus of claim 19, wherein the position sensor is an optical position sensor.
 25. The apparatus of claim 19, wherein the processor further compares the air content to a predetermined value.
 26. The apparatus of claim 25 further comprising an alarming device connected to the processor, wherein the processor activates the alarming device when the air content equals or is beyond the predetermined value.
 27. The apparatus of claim 26, wherein the processor prevents fluid from leaving the central chamber when the air content equals or is beyond the predetermined value.
 28. The apparatus of claim 27 further comprising an outlet valve controlling fluid flow out of the central chamber, wherein the processor closes the outlet valve and turns off the motor when the air content equals or is beyond the predetermined value.
 29. The apparatus of claim 19, wherein the processor calculates the air content at different time points and generates an accumulated air content over a period of time.
 30. The apparatus of claim 29, wherein the processor further compares the accumulated air content with a predetermined value.
 31. The apparatus of claim 30 further comprising an alarming device connected to the processor, wherein the processor activates the alarming device when the accumulated air content equals or is beyond the predetermined value.
 32. The apparatus of claim 31, wherein the processor prevents fluid from leaving the central chamber when the accumulated air content equals or is beyond the predetermined value.
 33. The apparatus of claim 32 further comprising an outlet valve controlling fluid flow out of the central chamber, wherein the processor closes the outlet valve and turns off the motor when the accumulated air content equals or is beyond the predetermined value.
 34. The apparatus of claim 19, wherein the processor calculates the base volume change by calculating a median base volume change of more than one sample of an infusion fluid.
 35. The apparatus of claim 34, wherein the processor calculates the bases volume change by calculating a median base volume change of 11 contiguous samples of an infusion fluid.
 36. The apparatus of claim 34, wherein the sample fluid is a sample of the infusion fluid.
 37. A method of measuring air content in a sample fluid comprising: determining a base volume change between zero or a negative pressure and a positive pressure in a chamber; determining a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid; and determining air content of the sample fluid by comparing the sample volume change and the base volume change; and wherein the sample fluid is a body fluid of a mammal.
 38. The method of claim 37, wherein the mammal is a human.
 39. The method of claim 37, wherein the body fluid is blood.
 40. The method of claim 37, wherein the body fluid is blood in a vessel selected from the group consisting of artery, vein, and capillary.
 41. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and wherein the determining the air content comprises comparing a) a base volume change between zero or a negative pressure and a positive pressure in a chamber, and b) a sample volume change between zero or the negative pressure and the positive pressure in the chamber with a sample fluid.
 42. The method of claim 41, wherein the determining the air content comprises the sample volume change minus the base volume change.
 43. The method of claim 41, wherein the determining the base volume change comprises determining the base volume change based on one or more sample fluids and selecting a lower than average value as the base volume change.
 44. The method of claim 41, wherein the determining the base volume change comprises determining a median base volume change based on more than one sample fluid.
 45. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and wherein the determining the air content comprises determining an accumulated air content of more than one sample fluid over a period of time.
 46. The method of claim 45 further comprising comparing the accumulated air content to a predetermined value.
 47. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and adjusting a fluid flow rate through the chamber based on an air/fluid content proportion in a given stroke of the assembly to compensate for the air content detected in the sample fluid.
 48. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and determining an effective amount of fluid that is pumped out of the chamber based on the determined air content.
 49. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and wherein the determining comprises determining a position of the assembly.
 50. The method of claim 49, wherein the assembly comprises a cassette piston.
 51. The method of claim 49, wherein the chamber is part of an infusion pump.
 52. The method of claim 49, wherein the determining the position of the assembly comprises determining the position of the assembly using a position sensor.
 53. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and wherein the sample fluid is a sample of the infusion fluid.
 54. A method of measuring air content in a sample fluid comprising: providing a chamber having an inlet valve, an outlet valve, and an assembly connected to a pressure source operable to draw and pump the sample fluid into and out of, respectively, the chamber; determining air content of the sample fluid based on a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber; and wherein the sample fluid is a body fluid of a mammal.
 55. The method of claim 54, wherein the mammal is a human.
 56. The method of claim 54, wherein the body fluid is blood.
 57. The method of claim 54, wherein the body fluid is blood in a vessel selected from the group consisting of artery, vein, and capillary.
 58. An apparatus for air content measurement of a sample fluid comprising: a central chamber; an assembly moving in and out of the central chamber; a pressure source connected to the assembly for generating a negative pressure in the central chamber and drawing fluid into the central chamber via the assembly, and for generating a positive pressure in the central chamber and pumping fluid out of the central chamber via the assembly; a position sensor connected to the assembly, wherein the position sensor determines a position change of the assembly; and a processor operably connected to the position sensor for receiving the position change of the assembly, and the processor being operable to determine an air content of a sample fluid in the central chamber based on the positional change of the assembly.
 59. The apparatus of claim 58, wherein the processor is operable to determine an air content of the sample fluid based on the positional change of the assembly corresponding to a volume change of the sample fluid as it transitions between being under a negative pressure and a positive pressure within the chamber.
 60. The apparatus of claim 58, wherein the processor is operable to determine an air content of the sample fluid based on the positional change of the assembly by calculating a base volume change between zero or a negative pressure and a positive pressure in the central chamber, and a sample volume change between zero or a negative pressure and the positive pressure in the central chamber with a sample fluid.
 61. The apparatus of claim 60, wherein the processor is operable to calculate the base volume change by calculating a median base volume change of more than one sample fluid.
 62. The apparatus of claim 61, wherein the processor is operable to calculate the base volume change by calculating a median base volume change of 11 contiguous fluid samples.
 63. The apparatus of claim 61, wherein the sample fluid is a sample of an infusion fluid.
 64. The apparatus of claim 58, wherein the assembly is a cassette piston.
 65. The apparatus of claim 58, wherein the central chamber is part of an infusion pump.
 66. The apparatus of claim 58, wherein the position sensor is an optical position sensor.
 67. The apparatus of claim 58, wherein the processor is operable to compare the air content to a predetermined value.
 68. The apparatus of claim 67 further comprising an alarming device connected to the processor, wherein the processor is operable to activate the alarming device when the air content equals or is beyond the predetermined value.
 69. The apparatus of claim 68, wherein the processor prevents fluid from leaving the central chamber when the air content equals or is beyond the predetermined value.
 70. The apparatus of claim 69, wherein the processor is operable to calculate the air content at different time points and generate an accumulated air content over a period of time.
 71. The apparatus of claim 70, wherein the processor is operable to compare the accumulated air content with a predetermined value.
 72. The apparatus of claim 71, further comprising an alarming device connected to the processor, wherein the processor is operable to activate the alarming device when the accumulated air content equals or is beyond the predetermined value.
 73. The apparatus of claim 72, wherein the processor prevents fluid from leaving the central chamber when the accumulated air content equals or is beyond the predetermined value.
 74. The apparatus of claim 58, wherein the sample fluid is a body fluid of a mammal.
 75. The apparatus of claim 74, wherein the body fluid is blood.
 76. The apparatus of claim 75, wherein the body fluid is blood in a vessel selected from the group consisting of artery, vein, and capillary. 